"The bottom line is that we're closely monitoring it," Capt. Darryn James, a spokesman for U.S. Pacific Fleet, told the Honolulu Star-Advertiser. "I think it's important for us to say that we can assure the public that we've taken all precautions necessary to protect critical information. We're not surprised that it's there."

The appearance of the spy ship does raise questions, however, as the People's Liberation Army Navy (PLAN) already has four ships taking part in the exercise at the invitation of the United States. Beijing's participation in the exercise is a first, although legal barriers have precluded its sailors from being in drills that would reveal U.S. military secrets, according to the Wall Street Journal.

China's uninvited fifth ship — a Type 815 Dongdiao-class intelligence vessel named Beijixing — is operating a safe distance away from the Hawaiian coast and the other 50 ships taking part in the exercise "in accordance with international law," James told the Advertiser.

USNI has more:

Monitoring electronic signals and communications from rivals exercises is nothing new. The U.S. and then-Soviet navies were famous for stalking one anothers exercises to gain intelligence information. China often accuses the U.S. of doing the same in areas like the South China Sea.

Adversaries and allies both can learn much from monitoring and analyzing electronic signals from a plane or a warship — say the frequencies in which an air defense radar operates. Likewise the communications between ships and aircraft can tell an adversary procedures of how a navy operates.

Whatever intelligence China may be trying to glean from the more than 20 nations taking part in the exercise may set the country up to be disinvited to the next one. While a U.S. Navy spokesman told the Journal participants are usually invited back, at least one member of congress is not happy with the apparent intelligence gathering.

"Now we learn they chose to disrespect the 20 other international participants by sailing an intelligence gathering ship directly into the middle of the exercise," Rep. Randy Forbes (R-Va.), chairman of the House Armed Services Subcommittee on Seapower and Projection Forces, told USNI. "It is clear China is not ready to be a responsible partner and that their first trip to RIMPAC should probably be their last."

The RIMPAC exercise runs from June 26 to Aug. 1 around the Hawaiian islands and southern California.